Neutrons are uncharged elemental particles which do not ionize matter as they pass through it. Accordingly, the presence of neutron particles is difficult to detect. Thermal neutrons are produced by splitting atoms such as Uranium 235 in a nuclear reactor and slowing the velocity of the fissioned neutrons through collisions with some moderating material. Neutron detection in the prior art is typically performed with either gas detectors or scintillators. A Geiger counter is a conventional gas detector for detecting neutrons. The Geiger counter is a gas filled tube which may be filled with 3He or with BF3, but has limited utility since it is both bulky and expensive to manufacture. Moreover, the Geiger counter requires repeated calibration.
In scintillation detection, the interaction of neutrons with the detector scintillation material occurs within the material, while the detection occurs separately or at a distance by some other detection technique. Scintillator detection devices are based on the principle of scintillation, which is an indirect process in which the interaction of neutrons with a detector scintillation material generates light which, in turn, permits light detectors to be used from which the level of neutron presence can be established. However, the light detectors need to be sensitive to the wavelength of the light. Otherwise, an emulsion film must be used. Because optics cannot gather all of the light and some of the light is reabsorbed by the scintillating material, thus the use of scintillation detectors for detecting neutrons is inefficient. Furthermore, light detectors have an inherent sensitivity limit to all wavelengths.
A promising method to detect neutron has recently emerged, i.e., semiconductor (solid state) detection. This detection method employs a semiconductor that is neutron sensitive, and in particular, thermal neutron sensitive, with the detection and interaction (of neutrons and the detector material) both occurring within the neutron-sensitive material. One material under consideration is pyrolytic boron nitride (or “pBN”).
Pyrolytic boron nitride is known in the art, e.g., as formed by chemical vapor deposition using a process described in U.S. Pat. No. 3,182,006, the disclosure of which is herein incorporated by reference, involving introducing vapors of ammonia and a gaseous boron halide such as boron trichloride (BCl3) in a suitable ratio into a heated furnace reactor to deposit boron nitride on the surface of an appropriate substrate such as graphite. The boron nitride is deposited in layers and when separated from the substrate forms a free standing structure of pBN.
Pyrolytic boron nitride (“pBN”) is anisotropic and has a hexagonal crystal lattice. Most boron nitride made by chemical vapor deposition (CVD) is composed of hexagonal crytallites in which the a- and b-axes are predominantly oriented parallel to the deposition surface. The hexagonal structure and preferred orientation impart highly anisotropic properties to the pBN. Because of symmetry, the a- and b-axes are equivalent, so it is convenient to describe pBN as having only two sets of properties, i.e., in the ab direction and in the c direction. In a single crystal of BN, the ‘a or b planes’ are perpendicular to the layers. In pBN, the ‘a or b planes’ have no preferred orientation except in the direction normal to the deposition layers. The crystographic planes, such as the c plane, are normal to their axes, so that the c plane in pBN is predominantly parallel to the deposition layers. Since the pBN deposits are for practical purposes limited to a few mm thick, the edge surface area is small in comparison with that attainable on the deposition surface.                pBN typically contains roughly about 10 atomic % boron-10 (10B) isotope (or about 8.5% by weight) which has a large cross-section for thermal neutrons, allowing pBN to be used in a solid state thermal neutron detector, in which a direct electrical signal is formed proportional to the alpha particles generated from the interaction of the colliding neutrons with the boron-10 isotope in pBN. Attempts have been made in the semiconductor detector prior art to capture neutrons using a pBN detector fabricated in a conventional fashion, and oriented to collect neutrons through the deposition layers, i.e., the predominantly c-axis direction, but have yielded poor results.        
In U.S. Pat. No. 6,624,423, Applicants have surprisingly found that the electrical resistivity of pBN, in undoped form, is highly anisotropic and its value in a direction parallel to the plane is lower than its value in the perpendicular direction. Hence, by applying electrodes in the direction normal to the a-b plane (parallel to the “c” direction), a neutron detector can be constructed having a significantly increased sensitivity to thermal neutrons. As illustrated in FIG. 1(a), electrical contacts are applied to the opposing edge surfaces 12 of the pBN structure 10. FIG. 1(b) is an illustration as this prior art reference with a transverse strip or slice of pBN having a thickness “t” being cut from a plate of pBN. Contacts are then applied on the opposing face of the pBN cut, i.e., on the plane 2 and on the opposing face of plane 2 created by the cut. FIG. 1(c) is another illustration of this prior art reference with a transverse strip or slice of pBN being cut from the pBN plate. Contacts are then applied onto plane 3 and the opposing face of plane 3 as created by the cut, with the first contact being a thickness “t” away from the second contact.
In the present invention, Applicants have discovered that although the electrical resistivity of pBN in the perpendicular direction (parallel to the “c” direction) is higher than the direction parallel to the plane for neutron detectors employing pBN, the value can be reduced by doping the pBN with one of C, Si, or Ge and optionally with other dopants including oxygen. Thus, in these neutron detectors, electrodes can still be applied in the direction normal to the “c” direction (parallel to the c plane), facilitating the construction of neutron detectors. As shown in FIG. 1a, doped pBN of the present invention allows electrical contacts to be applied to the two opposing surfaces 11 of the structure 10. The neutron detector of the present invention retains all the advantages of a compact solid state detector that responds strictly to neutrons and is not affected by gamma rays.